1. Field of the Invention
The present invention relates generally to recuperators for gas turbine engines. More particularly, the present invention relates to microturbine recuperators formed as annular arrays of recuperator core segments having internal compressed air channels with inlet and outlet ports.
2. Description of the Prior Art
Microturbines are small gas turbines used for small-scale power generation at one point in a distributed network or at a remote location. These power sources typically have rated power outputs of between 25 kW and 500 kW. Relative to other technologies for small-scale power generation, microturbines offer a number of advantages, including: a small number of moving parts, compact size, light weight, greater efficiency, lower emissions, lower electricity costs, potential for low cost mass production, and opportunities to utilize waste fuels.
Recuperator technology allows microturbines to achieve substantial gains in power conversion efficiencies. A conventional microturbine achieves at most 20 percent efficiency without a recuperator. However, with a recuperator, the efficiency of microturbine power conversion efficiency improves to between 30 percent and 40 percent, depending on the recuperator's effectiveness. This increase in efficiency is essential to acceptance of microturbine technology in certain markets and to successful market competition with conventional gas turbines and reciprocating engines.
Capstone Turbine Corp., the assignee of the present invention, has employed annular recuperators in 30 kW microturbines. These 30 kW microturbine engines are described in Treece and McKeirnan, “Microturbine Recuperator Manufacturing and Operating Experience,” ASME paper GT-2002-30404 (2002), the details of which are incorporated herein by reference. Capstone has also developed and marketed 60 kW microturbines having similar annular recuperators. Commercial operating experience with Capstone's 30 kW and 60 kW microturbines has shown that annular recuperators perform well in these microturbines. The annular recuperators are more resilient to thermal cycling and have less total pressure drop as compared to box-type recuperators.
FIG. 1 shows the schematic diagram of a prototypical Capstone Microturbine. The airflow enters and exits the recuperator in a radial direction and the gas flows in an axial direction of the engine. The construction of the individual recuperator core segments of the C30 and C60 microturbines previously sold by the assignee of the present invention have included a pair of sheets of fin fold stainless steel material assembled with a plurality of spacer bars located between the sheets of material and including external stiffener bars, all of which are welded together in a suitable arrangement and have assembled therewith corrugated air inlet and outlet manifold inserts and gas side manifold inserts.
U.S. Pat. Nos. 6,112,403; 6,158,121; and 6,308,409 disclose recuperator core segments similar to those previously used by Capstone.
Other general background information on the state of the art of recuperator design for gas microturbines is found in the following: (1) McDonald “Gas Turbine Recuperator Technology Advancements”, presented at the Institute of Materials Conference on Materials Issues in Heat Exchangers and Boilers, Loughborough, UK, Oct. 17, 1995; (2) McDonald, “Recuperator Technology Evolution for Microturbines”, present at the ASME Turbo Expo 2002, Amsterdam, the Netherlands, Jun. 3–6, 2002; (3) “Ward and Holman”, “Primary Surface Recuperator for High Performance Prime Movers”, SAE paper number 920150 (1992); and (4) Parsons, “Development, Fabrication and Application of a Primary Surface Gas Turbine Recuperator”, SAE paper 851254 (1985).
As a part of the US Department of Energy's Advanced Microturbine System (AMTS) Project, the assignee of the present invention developed a 200 kW microturbine engine with annular recuperator. The goals of the AMTS Project were to achieve: (1) 40/45 percent fuel-to-electricity efficiencies; (2) capital cost of less than $500 per kW of rated output power; (3) reduction in NOx emissions to less than 9 parts per millions; (4) mean period of machine operation between overhaul of several years; and (5) greater flexibility in types of usable fuels.
There is a continuing need for improvements in recuperator technology for microturbines, and particularly for recuperators suitable for use with larger microturbines such as the 200 kW microturbine developed by the assignee of the present invention. In particular, improving the efficiency of the radial distribution of compressed air within the recuperator core segments will allow use of recuperator core segments having a greater radial width to axial length ratio while maintaining a high level of heat exchanger effectiveness.